The San Francisco Municipal Transportation Agency is still waiting for the guaranteed federal funding that will allow it to start digging the 1.6-mile Central Subway tunnel to Chinatown, but that's not slowing major construction to prepare for the excavation.

And that will mean a summer (and beyond) full of transit and traffic detours, delays and headaches.

On Tuesday, crews from joint-venture contractor Barnard Impregilo Healy will start digging up nearly a square block on Fourth Street between Bryant and Harrison streets and building a "launch box," the point at which two tunnel boring machines will start chewing the twin-bore tunnels next year.

Preliminary work on headwalls, 100-foot-deep walls that will stretch across Fourth Street at the site of the Yerba Buena/Moscone Station, began last week.

In the coming months, construction of headwalls will take place on Fourth Street between Folsom and Howard streets and on Stockton Street between Market and Geary streets. Workers will also drill holes to monitor ground stability and inject a mud-like grout into the soil where needed. Utility relocation work in the Union Square area will continue through the summer.

Of course, several Muni lines and streets will be affected by the construction. The 30-Stockton and 45-Union/Stockton lines will continue the new routes they moved to in January; the 8X/8AX/8BX-Bayshore Express and 91-Owl will be re-routed in July, and the temporary 8 shuttle route will be eliminated. Passengers on the 38/38L-Geary routes should be prepared for delays in the Union Square area.

During construction of the launch box, at least two lanes on Fourth Street will stay open at all times, which, of course, means that two will often be closed. All ramps to Interstate 80 will remain open during the day but could be closed at night. Needless to say, delays are expected and drivers are advised to steer clear of the area. Parking restrictions will also be in effect, and the sidewalk will be closed on one side of the street at a time.

Similarly, work on the Fourth Street headwalls will close two lanes between Folsom and Howard streets at times as well as sidewalks on the west side of the street. Access to Clementina Street between Fourth and Fifth streets will be restricted at times. Parking will be restricted in the area.

Around Union Square, detours will stay in place but taxis, Muni buses and tour buses probably will be banned from Stockton Street between Market and Post, where private vehicles are already prohibited.

Meanwhile, MTA officials are still awaiting word from the Federal Transit Administration on an anticipated federal funding guarantee worth $942 million, the bulk of the $1.6 billion cost. But Paul Rose, MTA spokesman, said the agency expects the federal agency to approve the agreement this month, sending it on to the secretary of transportation's office and then to Congress. It's expected to be approved in early fall.

Critics, including former Supervisors Quentin Koppand Aaron Peskin, have argued that the project costs too much and would be underused. They're campaigning to kill the federal funding, and have urged the MTA to stop preliminary construction while the money is still uncertain.